Process for mechanically producing coarse crystalline deposits



March 31, 1935.] R, SIEGLER 2,035,9g@

PROCESS FOR MECHANICALLY PRODUCING COARSE, CRYSTALLINE DEPOSITS FiledApril 6 1934 flowrlo/v 51735565,? ('00; we Am [NIT/AL P014519 32 IVA012mm l 5 k M L I 6 (9 [15A 7' [/vsuL A TING A SING ,4. Mar/45p ZIQUOI?Jr? verifier- Patented Mar. 31. 1936 'UNETED STATES PATENT OFFICEPROCESS FOR. MECHANIOALLY PRODUCING COARSE CRYSTALLINE DEPOSITS RobertSiegler, Stettin-Pommerensdorf,

Ger-

Application April 6, 1934, Serial No. 719,415 In Germany January 3, 19332 Claims.

According to the present state of the art if it is desired to obtain afine crystalline deposit and individual crystals upto about pea size thecrystallization operation is carried out with the liquid in motion,whilst in order to obtain coarse crystals the crystallization isefiected from an absolutely tranquil liquid.

For carrying out tranquil crystallization a comparatively large.quantity of solution is required, and a correspondingly large vesselcapacity for the cooling. The growth of large crystals requires a longtime, usually days and Weeks. The completion of the deposition ofcrystals is followed, in separate operative stages, by the separation ofthe crystal deposit from the mother liquor, crushing, drying and sortingout of the large crystal lumps formed by crystals growing together. Ithas long been impossible directly to obtain large crystals fromsolutions by tranquil crystallization or crystallization with movingsolutions while avoiding these bothersome manipulations.

The present process fills the gaps existing in the crystallization artas regards the crystal size of 7 crystal deposits prepared mechanicallyand in tranquility. For carrying out the process the basic conditionsfor the growth of large crystals, viz. time and tranquillity, areapplied to crystallization with motion. This is attained by creating alarge cooling surface in order to accelerate the crystallization, butduring the cooling of the solution any motion of the same with respectto the cooling walls of the crystallizer is limited as far as possible.

In order to carry out the process an endless band or belt guideduniformly over rollers is used, the upper-part of said band beingmounted so that it forms a trough. The trough formed by mounting in thisway may be cooled on all sides by gas and also from underneath byliquid. The cross section of the trough may be suitably formed accordingto the nature of the mounting employed; the cross section is preferablylike the segment of a circle, or trapezium-shaped, or parabolic. Inorder to produce crystal deposition which is as uniform as possible thefiat trapezium shape is to be preferred. The circle segment formrequires the simplest under-support. The parabolic cross section isproduced by suspending the transport belt at the edges, theunder-supports being wholly dispensed with.

Since at the reversal points the drive of the conveyor belt is effectedfrom the inside, the upper side of the belt is naturally stretched outstraight and consequently must be artificially shaped to acommodate agreater quantity of liquid. This is most simply efiected by insertingrollers which press the transport belt against the roller undersupportsor the suspension. Belt and rollers are protected when required bysuitable strippers which keep the first rollers free from crystal de- 5posit or remove the resulting crystal deposit from the belt prior to theend rollers in order to avoid the said deposit being spoiled by therollers.

The crystallization operation is perfectly uniform and automatic. Owingto the very slow mo- 10 tion of the conveyor belt the power consumptionis extraordinarily small. The endless belt, suitably shaped at theedges, may be made of rubber or some suitable metal, according to thechemical properties of the solution to be crystallized.

The accompanying drawing shows schematically by way of example anapparatus for carrying out the process according to the invention. Inthe drawing Fig. 1 shows a longitudinal section through the apparatus,

Fig. 2 shows a cross section through a belt mounted so as to form atrough which in section is shaped like a segment of a circle having acorresponding under-support,

Fig. 3 shows a cross section through a belt mounted so as to form atrough which is trapezium shaped in section, having rollerunder-supports (of. Fig. 1), and

Fig. 4 shows a cross section through a belt mounted with parabolicsuspension.

The suspension shown in Fig. 4 is adapted for the production ofparticularly coarsely granular material, as well as for use when watercooling is to be employed.

The upper part of the belt I is brought into the necessary trough shape3, which ensures the maintenance of a certain liquid level throughoutthe whole length of the trough, by means of inserted rollers 2. In thisway the formation or growth of crystals corresponding to the depth ofthe liquid is rendered possible. The shape of the cross section of thetrough affects the uniformity of the crystal deposit and the production5 of a uniform crystal deposit of definite size is best ensured by theuse of a cross section of flat trapezium shape (Fig. 3).

In order to prevent undesired crystallization occurring at the placewhere the solution runs directly on to the band, the latter is protectedfrom heat loss at this place by an enclosing structure 4 made of wood orthe like. Water cooling is preferably accomplished by dipping thecrystallization trough into a vessel, likewise troughshaped, adapted tothe shape of the crystallization trough, the cooling liquidadvantageously moving in counter-current to the solution. Thespontaneous surface cooling of the solution by the outer a'ir'may bepromoted by means of a fan or blower 5; it is also possible to cool downthe belt at its under surface in the same way. The removal of thecrystal deposit from the belt is effected automatically by change inshape of the trough e. g. its conversion into a flat belt and thecurvature of the same over the driving or conveying pulleys 6. Residuesof crystal deposits and mother liquor still adhering to the belt areremoved by a scraper I. The guiding rolls for the belt are indicated by8, the driving pulleys at the commencing end by 9, the scraper at thecommencing end by I 0, whilst I [indicates a scraper before the secondinserted roller 2.

The substances to be crystallized (particularly in the case of thegrowth of crystal individuals) are obtained in the. characteristic formscorre sponding to their particular nature; for example, potassiumchlorate is obtained in plates, sodium sulphate and magnesium, sulphatein needles, that is to say, the salts are obtained with the'typical'characteristics of tranquil crystallization.

The size of the crystals isa function of the depth of the liquid layerin the belt and the nature of the cooling; by employing: Water coolingor air cooling, or a combination of both, the crystal size may be variedwithin Wide limits, as a result of which the individual crystallographicproperties. of the body are sharply brought out.

The continuous operative nature of the process is still preserved evenif. the supply of solutions istemporarily interrupted in order to. growparticularly large crystals.

It has already been proposed to use belts forming trough-shapedindividual containers for the discontinuous production of crystals, thecrystallization in the said containers proceeding tranquilly.- At theconclusion of the crystallization in order to remove the resultingcrystal deposit the shape of the belts, made partly of elastic orresilient material, was changed.

Such devices were still unsuited for the uniform uninterruptedproduction of coarse crystalline deposit by mechanical means.

What I claim is:-

1., A method of crystallizing a solution with continuous production of acoarse crystalline deposit consisting in supplying the solution to acontinuously moving surface which is temporarily given a trough-likeshape near the point at which the solution is supplied, and at a pointremoved from this in the path of the surface, changing the direction ofthe latter and simultaneously de-' priving, it of its trough-like shape,and continue ously removing'the crystals from said surface at the placewhere the surface changes its direction.

2. A method of crystallizing a solution with continuous production of acoarse crystalline deposit consisting in supplying the solution to acontinuously moving surface which is temporarily given a trough-likeshape near the point at which the solution is supplied, conducting thetrough shaped surface supporting the solution supplied solution, andthen changing the directionof the surface and simultaneously deprivingit of its trough-like shape, and continuously removing the crystals fromsaid surface at the place where the surface changes its direction. 7

ROBERT SIEGLER'.

30 past means adapted to promote cooling of the 7

